The present invention relates in general to an audio power amplifier, and more specifically to an amplifier comprising monolithic integrated circuits providing high output power using parallel class AB output stages.
Automotive audio systems typically receive electrical power from low voltage electrical systems. This results in various difficulties in attempting to provide high levels of amplification in the audio system for driving output loudspeakers. Various techniques have been employed to overcome the problem of inadequate voltage headroom being available for the output audio amplifiers. For example, output power to the speaker can be increased by using a specially designed low impedance speaker. In the prior art however, only class D amplifier configurations have been used to drive low impedance speakers because only such a high efficiency amplifier could provide the current levels necessary to drive a low impedance speaker when using a low voltage supply. However, class D amplifiers are relatively expensive for use in automotive audio systems.
It is also known in the prior art to employ a switching power supply in the audio system to raise the supply voltage for the output amplifier to thereby increase output power to the speaker. However, switching power supplies have the disadvantages of greatly increased costs and increased electromagnetic interference.
Especially in automotive audio systems, it is preferable to construct amplifiers using monolithic integrated circuits. IC's provide the lowest cost and require the least amount of space due to the reduced number of components in an amplifier. However, monolithic IC amplifiers have been limited in the amount of output power which can be provided at the voltages used in automotive electrical systems. The lack of sufficient power is especially a problem for subwoofer speakers for which a loud sound pressure level is desired.
Monolithic IC's are available with bridged outputs for increasing output power. In the bridged configuration, the opposite speaker voice coil inputs are each driven by separate amplifier stages which have their polarity inverted. More specifically, each amplifier stage is biased at about one-half of the supply voltage and each amplifier stage amplifies the input signal in relation to the bias voltage but in an opposite sense with respect to the other amplifier stage. Bridged outputs increase the power applied to the speaker without needing a negative or split voltage supply. However, bridged amplifiers have still failed to provide sufficient output power for many applications, including subwoofer applications.